Light olefins, defined herein as ethylene, propylene, butylene and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, light olefins are produced by cracking petroleum feeds. Because of the limited supply of competitive petroleum feeds, the opportunities to produce low cost light olefins from petroleum feeds are limited. Efforts to develop light olefin production technologies based on alternative feeds have increased.
Important alternate feeds for the production of light olefins are oxygenates, such as, for example, alcohols, particularly methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
The process for converting a feedstock, especially a feedstock containing one or more oxygenates, in the presence of a molecular sieve catalyst composition according to the invention, is carried out in a reaction process in a reactor, where the process is a fixed bed process, a fluidized bed process, preferably a continuous fluidized bed process, and most preferably a continuous high velocity fluidized bed process.
The reaction processes can take place in a variety of catalytic reactors such as hybrid reactors that have a dense bed or fixed bed zones and/or fast fluidized bed reaction zones coupled together, circulating fluidized bed reactors, riser reactors, and the like. Suitable conventional reactor types are described in for example U.S. Pat. No. 4,076,796, U.S. Pat. No. 6,287,522 (dual riser), and Fluidization Engineering, D. Kunii and O. Levenspiel, Robert E. Krieger Publishing Company, New York, N.Y. 1977, which are all herein fully incorporated by reference.
The preferred reactor types are riser reactors generally described in Riser Reactor, Fluidization and Fluid-Particle Systems, pages 48 to 59, F. A. Zenz and D. F. Othmo, Reinhold Publishing Corporation, New York, 1960, and U.S. Pat. No. 6,166,282 (fast-fluidized bed reactor), and U.S. patent application Ser. No. 09/564,613 filed May 4, 2000 (multiple riser reactor), which are all herein fully incorporated by reference.
In a preferred embodiment of oxygenates to olefins conversion, a fluidized bed process or high velocity fluidized bed process is employed which includes a reactor system, a regeneration system and a recovery system.
In order to optimize operation of oxygenates conversion to light olefins, it is desirable to control various parameters associated with the oxygenate to olefins conversion reactor. Such control can enhance oxygenate conversion and/or selectivity for prime olefins, especially for ethylene and propylene.
U.S. Pat. No. 6,166,282 to Miller teaches a process for converting oxygenates to light olefins in a fast-fluidized bed reactor and further observes that oxygenate conversion processes can be sensitive to reaction variables such as temperature, catalytic activity, and space velocity.
U.S. Pat. No. 5,952,538 to Vaughn et al. discloses an optimal range of space velocities which are suitable for oxygenates to olefin conversion.
Gayubo, et al, Ind. Eng. Chem. Res. 2000, 39, 292-300, disclose that in conversion to olefins, higher average reaction temperatures at a given coke level on the catalyst increases selectivity to ethylene.
U.S. Pat. No. 6,137,022 to Kuechler et al. discloses oxygenates to olefins conversion in the presence of silicoaluminophosphate molecular sieve-containing catalyst which maintains an optimal feedstock conversion between 80% and,99% under conditions effective to convert 100% of the feedstock when the reaction zone contains at least 33 volume percent of the silicoaluminophosphate molecular sieve.
U.S. Pat. No. 6,023,005 discloses the importance of maintaining optimal average coke levels on oxygenates to olefins conversion catalyst to effect improved lower olefin selectivity.
Control of the above-noted variables is shown as useful for optimizing performance of an oxygenates to olefins reactor. However, there are several problems encountered when attempting to select a control scheme for controlling space velocity, average reaction temperature, conversion of reactant and average coke level on catalyst. For example, measurement of the coke level on catalyst is difficult inasmuch as a sample of catalyst must be withdrawn and analyzed by a laboratory method. There does not currently exist a reliable means to continuously monitor the coke level on catalyst inside of a reactor. A similar problem exists with the measurement of the reactant conversion level. Control engineers generally prefer to use more reliable measured variables, e.g., temperature, for direct control of a process rather than conversion measurement devices.
Thus, a simple and effective control method is needed for controlling oxygenates to olefins reactor systems that does not rely directly on the measurement of variables that are difficult to obtain or which are inherently unreliable in their measurement. Furthermore, it would be desirable to provide a method for controlling oxygenate to olefins reactor systems which utilizes select manipulated and controlled variables so that the response in a measured variable of the control system occurs shortly after a change to the manipulated variable.